Facebook is a social networking service that allows users to post profiles and interact with the profiles of other users. It was founded in February 2004 and is currently reckoned to have 750 million active users. The site was founded by four Harvard computer science undergraduates, Mark Zuckerberg, Eduardo Saverin, Dustin Moskovitz and Chris Hughes.

Originally Facebook was only open to students of Harvard University, before those from other Ivy League institutions and Stanford University were also accepted, followed by high school students and then anyone over the age of 13. Facebook is named after the directories of students produced by American universities, especially at the beginning of each year to help students get to know each other. Zuckerberg is the current CEO and public face of the company and owns a 24% stake in it.

The majority of Facebook's revenue comes from advertising and Microsoft is their sole partner for serving banner adverts. Facebook has a lower click through rate (the amount of times an advert is clicked on) than much advertising on the web. There are a number of reasons why this might be so, including the young usership who are used to ignoring ads, the use of ad-blocking software and the nature of the media - people use it for writing messages, which can make adverts difficult to see. Many people also check the site on their smart phones, so even if they are using O2, they may well not see an ad for o2.co.uk when they are looking at their Facebook feed.

A Facebook user can create a profile with pictures and a list of personal information, including personal relationships, job, religious beliefs etc. Users can communicate with each other either by writing on a 'wall', a real time public space that is visible for visitors to see, or through messages which are akin to emails. Users can set their 'status' to tell others what they are doing or feeling at that moment, they can also 'poke' others, which means other users are told that they have been 'poked'. The site also has an instant messaging service. It is possible to sell things on Facebook via Marketplace, and to send virtual presents via Gifts.

In 2008 Facebook gained an Alexa ranking of 2, which as of August 2011 it still retains. The site is the most popular social networking site in the USA, Canada and the UK. The site has been banned, at least for a period of time, in China, Pakistan and Syria among other countries, and its instrumental influence has been claimed in the protests of the Arab spring, most notably in Egypt where a man named his newly born daughter 'Facebook'.